1. Field of the Invention
The invention relates to an encapsulated motion transducer, and more particularly to a transducer for measuring displacement and very small amplitude vibrations in hostile environmental conditions such as grease, oil, metallic sludge, corrosion, high ambient vibration, high temperature, electrical, and electromagnetic interference.
2. Related Art
Fiber optic devices for the detection and measurement of displacement and vibration have been disclosed by U.S. Pat. No. 3,273,447 to Frank and by U.S. Pat. No. 3,327,584 to Kissinger. The output of devices attributable to Kissinger, which have been commercially marketed, are proportional to target surface motion as well as target surface reflectivity. To sense and measure motion precisely with these devices it is necessary to ensure that the target surface reflectivity is constant while measurements are being taken.
It has been found that accurate dynamic measurements can not be made with unencapsulated fiber optic devices in environments where there is contamination of the target surface or of the optical path to the target surface. Other non-contact motion transducers, such as eddy current or capacitive types also suffer a degradation of performance when used in an environment that causes metallic based contamination to collect at the sensing tip. For example when using these unencapsulated devices to monitor bearing vibration in the manner disclosed in U.S. Pat. No. 4,196,629 (which is hereby incorporated by reference), it was found that bearings corrode in their housings and that bearing lubricant migrates towards the sensing area mixing with the corrosion products as it migrates. The mixing of corrosion products and lubricant creates a metallic sludge that degrades the performance of any transducer that is sensitive to metallic substances or is dependent upon a clear optical path to the target.
A surface-contacting fiber optic displacement transducer has been disclosed by Philips in copending application Ser. No. 748,084, filed Sept. 24, 1985, and is designed to overcome work surface reflectivity problems by encapsulating the fiber optic sensor tip. The elastomeric biasing means of those devices have been found to create distortions in the frequency response of light that is reflected from the sensing means reflective surface. A typical frequency response of the device using elastomeric biasing is shown in FIG. 1. The desired frequency response is a straight line as indicated on the figure. Distortions of the type shown in FIG. 1 significantly degrade the capacity of these devices to provide precise motion measurements at all frequencies of vibration. Furthermore, to minimize the amount of distortion, the force generated by the elastomeric biasing means must be kept very small. Thus, intimate contact between sensor means and target surface can not be maintained with these devices when vibrations are present which generate acceleration forces on the sensor tip which exceed the small elastomeric spring force. A loss of intimate contact can occur when the target vibrates excessively or when the device is installed in a moving vehicle that is subject to large accelerations.
The elastomeric biasing means disclosed in the above application has a restricted amount of motion that is 0.003 inches or only slightly more. This restriction prohibits the setting of an operational gap at what is known as the optical peak in the response curve of the devices attributable to Kissinger. The optical peak, a typical example of which is indicated in FIG. 2, is the region at the peak of the output curve where changes in the amplitude of reflected light are proportional only to target surface reflectivity changes and not to gap changes. The optical peak is thus the only point at which reflectivity of a target surface can be accurately and reliably checked for its absolute value. This is important because reflective surfaces can oxidize or otherwise degrade over a period of time, especially when subjected to elevated temperatures. The restricted range of motion of the device in the application also markedly increases the sensitivity of the device to installation errors thereby rendering the device much less practical to employ.
Many bearings are subject to extremes of operating temperatures. An example of this application would be small high speed turbomachinery with bearings located close to hot turbines. Normal bearing operating temperatures run up to 400 F. Mildly elevated temperatures are considered to be in the range 400-600 F. A limited number of special bearing applications extend to 1000 F and even higher. The above application does not provide considerations for operation at elevated temperatures.
Electrical noise and electromagnetic interference are problems frequently encountered with electrical sensing devices. These interferences are particularly troublesome when very low amplitude vibrations are to be measured. In the device disclosed by U.S. Pat. No. 4,196,629 to Philips low amplitude bearing vibrations are sensed and converted to bearing noise levels. That noise level reading is degraded when the sensing instrument self-noise and/or ambient noise interfere with the motion sensing device.
A prior displacement probe is illustrated in U.S. Pat. No. 4,171,645 to Miserentino, et al which includes a target in the form of a ball or planar member. The target is held in contact with a vibrating surface by gravity, a set of springs, a balloon device or a jet spray of gas. However, the Miserentino, et al probe lacks means for sealing the transducer elements from hostile environmental interferences, lacks material means for achieving successful operation at elevated temperatures, lacks coupling means to maximize the amount of light throughput, lacks means for overcoming ambient and self-generated noise, and it lacks a method for maintaining sensor contact with test objects in hostile vibration environments.
Other types of non-contact motion transducers that do not use light sensing means are also available commercially. Of those, eddy-current and capacitance sensing devices are very common. These devices are not sensitive to target surface reflectivity variations but their usefullness in hostile environmental conditions is significantly degraded by metallic contamination, by elevated temperatures, and by electrical and eletromagnetic interferences.